The Analogue Network

The analogue network or Public Switched Telephone Network (PSTN) still used by the majority of subscribers has developed over the years to include an increasing proportion of digital equipment and is a combination of digital and analogue technology. It will be referred to as analogue to distinguish it from the fully digital services which are becoming more widely used. The explanations below have been simplified and intended to give an overview which is accessible to all readers. For a more in depth explanation further reading will be required.

5.1 - Structure

5.1.1 - Exchange Network

The network has been organised on a hierarchical basis with the size and traffic handling capacity of exchanges being increased at higher levels. For purposes of illustration a three level hierarchy will be discussed here.

Figure 1 - The Three Layer Network Hierarchy

In Figure 1 a three level hierarchy is shown. The local exchanges are connected to the subscribers below them and to the larger concentrator exchanges above them. Each concentrating exchange will service many local exchanges below it and pass traffic on to the trunk exchanges above it. The trunk exchanges will then route calls to the appropriate trunk exchanges before passing the traffic back down through the levels until the local exchange makes the dialled telephone ring. The different functions may be housed in the same building, even the same physical rack. The distinction is more conceptual one than a physical one.

As the traffic is routed higher up the levels, higher capacity trunks are used (denoted by the thicker connecting lines) to carry the increased number of calls. These are now digital fibre optic links using Time Division Multiplexing (TDM) to merge many calls into a single connection. The larger the connection capacity the more calls can be carried.

The route for any given call will depend upon the distance. Local calls may take place within the local exchange and route back down to the dialled telephone if the call is nearby or may route to the concentrator and back out to a nearby local exchange to its destination. The longer the distance the call the higher up the hierarchy it will be passed.

5.1.2 - Subscriber Lines

Below the local exchanges are the subscriber lines. these will be connected to individual telephones or customer telephone systems. They are commonly twisted pair copper wire 0.5mm diameter for each line. each line can carry a single call.

5.1.3 - Party Lines

Party lines are rare today and will not be found very often. With party line connection two telephones are each connected to the same copper pair from the local exchange. When one of the telephones is taken off hook (the handset picked up) one wire of the pair is earthed and the local exchange can thus determine which of the connected devices it is. The call can then be made in the usual way. Whilst one of the telephones is using the line the other cannot make or receive calls.

5.2 - Routing

Routing of calls is performed by the network in response to dialling signals from the subscriber telephones connected to it. The workings of the telephone are described in the section on the Simple Telephone and methods of dialling are described in the following paragraphs.

Each connected telephone on the network has a unique address or telephone number. The number is used to determine the route the call must take to reach the called party. Figure 2 illustrates this process.

Figure 2 - Routing A Call

For clarity a simplified network has been represented. The caller dials the number 0617-208-994. The leading 0 tells the local exchange the call is to go out of the area. the 617 identifies the trunk exchange for the call. The 208 tells the trunk exchange which of its local exchanges is to get the call and the final part 994 identifies the destination telephone. If not busy, ringing is sent to the destination telephone and ring tone returned to the caller.

In practice the call may cross several layers of the hierarchy before emerging at its destination. Older exchanges worked using the stepping process as described above, modern exchanges wait for all the number to be dialled and then route the call using the whole number in a single step. This makes call set up much faster.

Dialling can be either pulse or tone the difference between these two types of signalling are described in the following paragraphs, along with ringing and call progress tones.

5.2.1 - Pulse Dialling

This is the oldest form of dialling. Being used from the advent of automatic exchanges at the end of the 1800s it is still in use today. Most modern equipment can produce or recognise pulse (or loop disconnect) dialling, however the new tone dialling system offers speed, flexibility, additional functionality and service access.

It is a series of breaks in the telephone line circuit which the exchange recognises as digits. In the original exchanges the breaks would be created by the stepping of the dial on the telephone and used to step the electromechanical switches at the exchange to the select the correct destination line. In modern equipment a semiconductor would control the breaks in response to the keypad digits pressed and the exchange will collect all the digits before selecting the route electronically.

Each number is represented by the same number of pulses. each pulse lasting 100ms. This 100ms is divided into a 60ms connection and a 40ms break. Dialling 1 would send one pulse, 2 would send two pulses and so on to 0 which sends 10 pulses. Between each digit is an interdigit time of approximately 700ms. Therefore the time to send digits depends upon the digits dialled. 1 taking 100ms to 0 taking 1sec. Added to this is the interdigit timer between each digit.
This continual making and breaking of the circuit gives the characteristic clicking in the earpiece of the dialling telephone.

5.2.2 - Tone Dialling

Tone dialling, also called Dual Tone Multi-Frequency (DTMF) or `TouchTone', is a relatively recent development. It is now accepted by virtually all exchanges and will eventually supersede pulse dialling altogether.

Each digit is represented by a combination of two tones from seven. Figure 3 shows a telephone key pad and the tones used.

	1	1	1	1
	2	3	4	6
	0	3	7	3
	9	6	7	3
	H	H	H	H
	z	z	z	z
697Hz
770Hz
852Hz
941Hz
Figure 3 Tone Dialling Matrix

Included in Figure 3 are the special keys A, B, C and D. These are used in some systems to perform special functions. They are included for completeness and will not be discussed here.

When a key is pressed the two tones which `cross' at the key are transmitted to the exchange. E.g. dialling a 1 will send 697Hz and 1209Hz together, 2 will send 697Hz and 1336Hz and so on. Each digit is represented by a unique combination of tones. DTMF digits can be sent and detected more quickly than pulse digits. A detection time of only 50ms with an interdigit time of 50ms means that any digit can be dialled in 100ms. Compared to a pulse dialled 1 taking 800ms and pulse 0 taking 1.7sec.

DTMF dialling is therefore much faster than pulse dialling. The tones as they are transmitted give a distinctive chirping sound in the earpiece as they are sent.

5.2.3 - Ringing

To indicate the presence of an incoming call the exchange must make the telephones connected ring. This is done using a ring generator at the exchange that sends a 75Vac signal between 20Hz and 47Hz to the subscriber end. This triggers the ringer in the telephone.

Ringing is sent to the called party first, then the calling party. This allows the exchange ring generator to make most efficient use of the power required to ring the handsets. It also means the caller has ringing around 2 seconds before the calling party hears it. This can lead to the called party answering before the calling party even hears a ring tone if they are quick.

5.2.4 - Call Progress Tones

Another important function of the exchange is to inform the caller of the progress, success and failure of their call. this is performed using a series of tones. Each is different and indicates a specific condition. These are:

Dial Tone	OK to proceed with dialling
Busy Tone	The called party equipment is in use
Ring Tone	The called party equipment is ringing
Unobtainable	The dialled number does not exist or cannot be connected

The precise nature of these tones varies between networks and systems. Modern equipment has replaced some with spoken messages giving a more friendly and understandable message.

 

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